Electric wave filter



D. C. ROGERS u j` :menue um FILTER Filed '14, 1942- ff/V704? 11:. masas,

cuando Nev. 2s, 194e Douglas Cecil Rogers, London, England, assignor to Standard Telephones and Cables Limited, London, England, a, British company Application October 14, 1842, Serial No. 462,026

Y l In GreatBritain October 15, 1941 1 'Ihe present invention relates to iiltexing ar rangements for ultrahighfrequency electric wave transmission or generating apparatus, and in particular to arrangements for preventing leakage of the high frequency into the power supply leads One of the problems presenting great difficulty in the design of apparatus for use on centimetro range wavelengths, is that of providing adequate filtering for the supply leads.

One method, described in British patent. specication Nn 541-380, which has been found effective, is the use of an arrangement resembling a condenser haring input and output leads at opposite ends of each foil. The condenser then becomes a four terminal network, and takes the form of a transmission line of low characteristic impedance. By this means the residual lead inductance found in the more conventional type oi condenser is made to contribute to the desired eli'ect, instead of being detrimental.

'Ihe present specication describes a method whereby the effectiveness of the arrangement described above can be considerably increased.

The method is suitable for use at wave lengths between about 1 centimetro and 1 metre.

According to the invention, there is provided an electric wave iilter for ultra high frequencies comprising a conductor in the form of an elongated thin metallic sheet divided into alternate wide and narrow portions arranged in close prox imity to, but electrically insulated from, anotherv conducting sheet According to another aspect, the invention comprises an electric wave lter for ultra-high frequencies comprising a series of tandem connected transmission lines of alternate low and high image impedance, each of the said lines comprising a pair of thin flat parallel sheet conductors arranged in close proximity to, but electrically insulated from, one another.

The invention will be better understood by reference to the following detailed description and to certain gures on the accompanying drawing in' which:

Figs. 1A. B, and C, show examples of the forms of transmission line conductors according to the invention; and Pig. 2 shows a lter circuit (not referred to in this specication);

Fig, 3 shows a portion of the conductor of Fig. 1A to define an elementary section of the filter;

Fig. 4 shows an equivalent block schematic diagram of the elementary section of Fig. 3; and

Fig. 5 is a. sectionel end view of the conductors forming the lter.

7 Claims. (CL v178-44) l.The arrangement described in specication No.

541,360 usually takes me ferm of a unn stnp of copper or other metal clamped between two earthed copper plates, and insulated with thin sheets of mica, or other dielectric material. The mothodof increasing the effectiveness of such a lilter according to the present invention is to use a metal strip cut to a partcula.l` shape such as is shown in Fig. 1A, or an equivalent conductor, in place ci' the plain rectangular strip previously used. It will he seen that the strip is thus divided into alternate wide and narrow sections, corresponding to short lengths of transmission line oi low and high characteristic impedance l5 respectively.

This arrangement can be shown from well known transmission theory to have properties similar to those of wave lters, in so far as it exhibits pass bands and attenuating bands, which can be made to cover given frequency ranges by suitable dimensioning of the conductor strip shown in Fig. 1A. A wide variety of shapes is, however, possible, two other examples being shown in Figs 1B and 1C, which produce substantially the same result as Fig. 1A. Fig. 1C

isa. convenient form because it enables the total length of the conductor strip to be reduced.

The chief advantage of the arrangement of the invention lies in its simple construction. The

construction of lters using the same underlying principle has previously been described. such nltcrs being of the coaxial line type: in this case the necessary changes of impedance are brought about by cylindrical metal blocks threaded on to the centre conductor.

By suitably proportioning the lengths and impedances of the various sections of the strip, it is'.possible to produce characteristics similar to those of conventional multiple band-pass or band elimination filters.

` Fig. 3 shows a portion of a strip similar to Fig.

lA comprising two wide sections and one narrow section between them. If the wide sections be supposed to be bisected by the dotted lines n and YY, then the vrhole strip may be regarded as made up of a number of elementary sections like that comprised between the lines XX and YY, joined in tandem. These sections will be all equal and symmetrical.

The strip may be made i-..to a iilter by mounting it parallel to a plain metal strip or tc another strip of the same form. with suitable insulation between. This filter can then be regarded as made up of a number of tandem connected transmission lines of alternatively high and low .fnnpemwnimmbedivicedmtossees of symmetrical sections each of which corre- I lows that spondstottxeportionhetweenthelinesXXand YY in Fig. 3.

Pig. 4 shows a block schematic circuit of the equivalent electrical network of the tilter corresponding to this portion of the strip. It com- I prises two symmetrical networks A and C each 'corresponding to half a wide portion and a. third symmetrical network B connected between them and corresponding to a whole narrow portion. A and C have image impedance Zi and image transier constant vr/2, and B has an image impedance Z2 and image transfer constant or. The image impedance oi the combination representing the elementary section is Z and the image transfer constant is o.

.Assuming that the transmission lines do not themselves introduce any transmission los, or in other words that i and e1 have no real components, it can easily be shown that zl/zn sin e sin a u) The pam frequency bands are obtained for those values of ai and 02 for which cosh lies between +1 and 1. In the suppression bands, the attenuation constant is given by:

cosh2 [cos t1 cos 0:- 1,5(21/22-5- Zz/Zi) Sin 91 Sin 02]: (2)

Il it he assumed that the electrical length of the elementary section XX, YY is fixed, or in other words that l1 +0z=20, then It can be shown that 5 is a maximum when O01=n1r/2. The most useful value of n is zero so that 0=e1=0z, and in these circumstances the maximum value of occurs when 0=(2n+ 1) 1r/2, and is given by It is, however, oi' more practica] value to determine the maximum attenuation per unit physical length of the strip than per elementary section. The attenuation per unit length .is given by:

The maximizn value of /a may be determined graphically for unous values of zz/Zl. For example, if Zz/Zi=10, which is a convenient practical value, tfollows that ww ,1.6) andthisisamaldmmnwhen 0==55Ppmx9 mately.

In this case the attenuation per section, which is given by cosh-1 (6.05 sinz 6 1), will be about 15.7 db. Il' x is the wave length, and x is the Physical length o! the elementary section, Fig. 3, then:

Thus it 0:55' and l=10 cm., then :=3.05 cm., and a lllter with ten elementary sections would have an attenuation of 157 db and would be jus over a foot long.

It can be shown 'that when 0=0;=:, the image impedance Z of the clem mt is given by When designing a lter to meet certain Elven requirements, the preferred procedure is to employ sections having maximum attenuation per unit physical length, and to choose 9:61:62.. The maximum value of Zz/Z1 which can reasonably be used will be decided from practical consolder-ations of the construction of the lter and will depend'emong other things on the amount of current the lter must be designed to carry. The corresponding attenuation per section ls determined by finding from Equation 5 for the given value of Zz/Zi the corresponding value of 0 for maximum attenuation per circuit length and obtaining thegconesponding value of per elementary section from Equation 3. This can best be done graphically by constructing a curve relating to /Zi from these two equations. I'his will give the number of sections required for any given total attenuation. The physical length x of the elementary section depends on the maximum wave-length to be attenuated. If this is x, then from Equation I the length :c may be obtained by substituting the previously found value of 0. Then /2 gives the length of the wide and narrow portions of the strip. A graph can oe constructed if desired to obtain a: directly from Zc/Zi, for the assumed attenuation conditions.

It can be shown that the attenuation for a. certain band of wave-lengths less than the chosen maximum wave-length will be equal to or greater than that of the maximum wave-length; and that the vridth of this band increases as Zz/Zi is increased.

In the above explanation it has been assumed that the effective dielectric for the section m, YY is air. If, as will generally be the case, the strips are insulated with a material having a dielectric constant K, then the length x determined from Equation 'I should be divided by VK. so .that the insulating material is an advantage because it makes the filter shorter.

It will be evident that the narrow portions of the lter may be bisected instead of the wide portions to form 'the elementary section. The discussion above will not be affected except that at the ends of the filter (assuming it is terminated in a half section) the image impedance Z will be given by an equation the same as (8) except that the left hand side is replaced by By using a strip having the form of Fig. 1C, the lter is shortened, as already explained; and if the clearance between the Wideand narrow portions is not so small that the capacity between them becomes appreciable. the formulae' explained above will be found to hold with sullicient accuracy.

An alternative improved method of construction of the filter of the invention is to deposit the metal directly on one or both sides of the dielec. tric sheet. Thus, copper may be plated directly. on to mica, for example, by rst', spraying thereon through a. stencil a colloidal graphite solution such as that known by the registered trade-mark Aquadag to form a conducting base. Alterna tively silver may be baked on to mica in a manner similar to that used in the construction of silver.

ceramic condensers. vIn this cose, the silver paste may be applied by means of a rubber stamp.

Experimental lters have been made by this method, and it is found that the thin layer of metal so formed is quite capable of carrying direct currents in the order ci one ampere. A further advantage is that the metal is in more intimate contact with the dielectric, thus increasing the capacity per unit lenen, thereby giving a lower input image impedance (a desirable fee ture when filtering from high impedance sources) The filter may be constructed in a form balanced or unbalanced to ground. In the rst case it may consist ci two identical strips of any of the forms shown in Figs. LA, 1B or 1C, (or any other like forms exhibiting wide and narrow portions) arranged to register on either side o! an insula.- ing strip. In the second case, `when one of the conductors will be at ground potential, this ccnductor would preferably he a plain rectangular strip of area suicient to cover the formed strip; and could conveniently be, for example, part of the screen of the apparatus in which the lter is to be mounted. In this case, for example, a thin strip of insulating material having the formed 25 conductor deposited on one side would be fixed to the screen, suitable input and output terminals in contact with the metal layer being provided at the ends of the strip; a suitably insulated plain earthed metal strip is preferably mounted also outside the conductor formed on the dielectric to serve as a screen. Allowance must of course be made for this extra Strip as it will roughly halve the impedance of the lter.

It will be clear that the strips may he arranged 35 in various other ways.

What is claimed is:

I. An electric wave lter for ultra high fre quencies comprising a nrst conductor in the form oi' an elongated thin metallic sheet divided into alternate Wide and narrow portions all of suhstantially the same physical length, a second con. dnctor in the form of a sheet, and means for insulatingly spacing the said nrst and second conductors from one another.

2. An electric wave filter for ultra high frel quencies comprising a series o! tandem connectedV transmission lines all of substantially equal electrical length and alternately of high and low image impedance, the said linc-s each comprising' a. pair of thin, hat. parallel sheet conductors in close proximity, but electrically insulated.

3. An electric wave filter for ultra high frequencies comprising a iirst conductor in the form of an elongated thin metallic sheet divided into alternate rectangular wide and narrow portions all of substantially the same physical length, a second conductor in the form of a sheet, and means for insulatingly spacing the said rst and second conductors from one another.

4. An electric wave lter for ultra high freshaped portions in registration with one another.

5. A lter as claimed in claim 3 in which the said wide and narrow portions are symmetrically arranged with respect to the centre line of the sheet.

6. An electric wave lter for ultra. high frequencies comprising an elongated metallic sheet conductor in the form of rectangular portions spaced apart, and narrow portions connecting dl-4 agonally oppositely facing corners of the said rectangular portions, a second conductor in the form of a. sheet, and means for insulatlngly spacing the said first and second conductors from one another.

1. An electric wave iilter for ultra. high frequencies comprising a series of tandem connected transmission lines of substantially equal electrical length and of alternate high and low image irnpedance, the said lines each comprising a pair of thin at parallel sheet conductors in close proximity but electrically insulated and of such electrical length in relation to the ratio of the said image impedance as to give a. maximum at- A tenuation per unit physical length of the saidl lter.

DOUGLAS cncn.. Roenes; 

